Bump pyyaml from 5.2 to 5.4

Bumps [pyyaml](https://github.com/yaml/pyyaml) from 5.2 to 5.4.
- [Release notes](https://github.com/yaml/pyyaml/releases)
- [Changelog](https://github.com/yaml/pyyaml/blob/master/CHANGES)
- [Commits](https://github.com/yaml/pyyaml/compare/5.2...5.4)

Signed-off-by: dependabot[bot] <support@github.com>

requirements.txt

@@ -10,7 +10,7 @@ lxml==4.4.2
 MarkupSafe==1.1.1
 python-dateutil==2.8.1
 python-frontmatter==0.5.0
-PyYAML==5.2
+PyYAML==5.4
 requests==2.22.0
 six==1.13.0
 unicode-slugify==0.1.3